This invention relates to rapid purging of fuel cell stack fuel flow fields and reactant manifolds, such as during startup and shutdown.
To achieve very high fuel utilizations, in excess of 95%, when operating a fuel cell stack on pure hydrogen, a cascade fuel flow field, illustrated in FIG. 1, comprises a plurality of groups 10-12 of fuel cells 13 arranged in flow-series relationship so that fuel from a source (not shown) passing through a fuel inlet valve 16 enters a fuel inlet manifold 17, flows through a first group 10 of fuel cells 13, then enters a first turn-around manifold 19, then flows through the second group 11 of fuel cells 13, thence through a second turn-around manifold 20 and through the third group 12 of fuel cells 13, to an exit manifold 22. For a typical 20 kilowatt fuel cell stack, the first group 10 has a large number of cells 13, which may be on the order of 95 or 105 cells, the second group 11 has a lesser number of cells 13, which may be on the order of 30-40 cells, and the third group 12 may have on the order of 5 to 15 cells. As is known, this assures that all of the cells get adequate hydrogen even with high hydrogen utilization, provided that the last group of cells 12 get adequate hydrogen.
In commonly owned, copending U.S. patent application Ser. No. 09/742,481, filed Dec. 20, 2000, now abandoned, it is shown that the more rapidly the fresh hydrogen-containing fuel flows through the anode flow field upon start-up, to displace the air therein, the quicker the hydrogen/air interface moves through the anode flow field, and the less time there is for the occurrence of corrosion of the platinum catalyst and catalyst support. In a similar fashion, it is known that the more quickly purge-air is passed through the anode upon shut-down, the less opportunity there is for hydrogen/oxygen interaction, which creates a safety hazard and may cause undesirably large voltage excursions in the cells, as described in commonly owned, copending U.S. patent application Ser. No. 09/742,497, filed Dec. 20, 2000, now abandoned.
With the cascade fuel flow field of the type described thus far with respect to FIG. 1, it is impossible to achieve rapid purging of the fuel flow fields since all of the purge gas must pass through a relatively small number of cells in the last cascade (the third group 12). This problem is a large contributor to the degradation of the first cascade (group 10) in fuel cell stacks employing cascade fuel flow fields, which in turn has impeded the adoption of the cascade design.
An alternative purge process may utilize inert gas, such as nitrogen, to purge the cell on shut-down; however, addition of an inert gas to a fuel cell system adds weight, cost and complexity which is unsuited for fuel cell powerplants used to power vehicles. Furthermore, the rapid deployment of hydrogen into the fuel flow fields upon startup, following an inert gas purge on shutdown, still remains a problem in a cascaded fuel flow field design.
Objects of the invention include: provision of a rapid purge in a fuel cell stack having cascade fuel flow fields; a fuel cell stack having the high-fuel-utilization advantage of a cascade fuel flow field without the purge problems heretofore associated therewith; an improved fuel cell stack utilizing cascade flow fields which has no unusual degradation of catalysts and other parts as a consequence of frequent shut-down and start-ups.
According to the present invention, a cascade reactant flow field of a fuel cell stack has at least one additional fuel inlet valve to provide inlet fuel directly to each cascade of the stack and at least one additional exhaust valve to remove fuel directly from each cascade of the stack.
The invention may be used not only for rapid deployment of fuel into the fuel flow field during start-up, but may also be used for a rapid purge of the fuel flow field with air during shut-down, provided the air is made available to the normal fuel inlet ports in an obvious fashion within the skill of the art.
Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing.